Friday, January 11, 2008

7.6 T. rex vision

Have you ever wondered how the dinosaurs saw when you look at their immense fossil heads with huge empty eye sockets?

Of course, all the soft ocular tissues are long gone, so it is impossible to know the gross anatomy of the eye, let alone how many kinds or the density of photoreceptors in the retina. Without the information, all studies are essentially best guesses.

T. rex and its relatives, the theropod dinosaurs, all have a huge head, small fore-limbs with sharp claws, and walked or trotted on powerful hind legs. Based on the position of the orbits, it is possible to estimate the overlapping visual fields of the two eyes. It turns out that two possibilities exist, one with a 20° overlap (similar to that of the crocodiles) and the other 45–60° (similar to that of the birds). T. rex belongs in the latter group with a 55° overlap (see image at top).

So what does this overlap mean? Most likely for stereopsis at close range. According to some paleontologists: if you are a predator as the T. rex, it is a good idea to see what you are biting at. T. rex is known to leave its well-placed tooth marks on its victims. On the other hand, a scavenger needs only to know where the meal lies; and a wider peripheral visual field is advantageous for scanning the horizon, in case some big dangerous looking T. rex is lurking nearby.

The above seem reasonable. The next assumption is big eyes must have excellent vision. In fact, there has been some exercise fitting an enlarged version of reptile or bird eye into the eye socket of a T. rex and project what its vision could be. Some claimed T. rex had 13 times better acuity than humans. From the retinal point of view, this appears unlikely. The eyes of a T. rex maybe several times larger than that of the humans, it is not the number but the density of the of photoreceptors that determines visual acuity. That is, if T. rex did have a fovea as in the humans. Perhaps it had multiple foveas each for a different visual function for all we know (the eagles have two, for example). Without an actual sample of the retina, it is simply not possible to draw any definitive conclusions.